The present invention relates generally to data links for telecommunications transmission facilities, and, more particularly, a ground fault detector for T1 span equipment.
Many telecommunications transmission facilities include a central office that transmits data signals over transmission lines to remote terminals and customer premises. The signals may be sent over two pairs of transmission lines in a digital format or differentially on two conductors, known as a Tip-Ring Pair.
The Bell telephone system in the United States, for example, has a widely utilized Digital "D" multiplexing pulse code modulation system. A "D" channel bank, for example, commonly provides multiple DS-1 signals that are carried on T1 span equipment. One pair of cables is provided for each direction of transmission.
The data, or "payload," signals to be sent over the transmission lines are sent differentially on the Tip-Ring Pair. A longitudinal, or common mode, direct current is applied to the simplex lead to power T1 line repeaters. Payload signals are transmitted by a telephone company central office to a series of regenerative repeaters or line repeaters.
The repeaters are spaced along the transmission lines approximately every 3,000-6,000 feet. When the first repeater receives the data from the central office, the signal may have degraded due to line losses, noise, interference and distortion. The repeater recognizes the presence or absence of a pulse at a particular point in time, and thereafter, if appropriate, generates a clean, new pulse.
The repeaters are powered by the transmission cable to generate the new pulses. The new pulses are transmitted by the line repeater along more cable to either another line repeater or to a Network Interface Unit ("NIU"). The NIU, in turn, transmits signals to and from equipment on the customer premises.
Some repeaters include a dead loop feature. In dead loop mode, a break in the transmission line or a disconnection of the customer's equipment from the NIU causes the line repeater to dead loop such that the powering current feed transmitted from the central office is routed back to the central office through the last repeater in the chain capable of being powered. Accordingly, the central office is advised of an abnormality along the transmission cable. The dead loop condition will be released if, for example, the line is repaired or the customer equipment is reconnected to the NIU.
The transmission facilities generally include a central office switch that provides an analog signal to a central office channel unit. The central office channel unit converts a two wire analog signal to a digital data signal transmitted on two pairs of cables. The digital data signals are transmitted to a remote transmission channel unit, or NIU. The digital data signal is then reconverted to an analog signal and provided, in analog form, on two a wire conductor, or Tip-Ring pair, to the customer premises.
Signals that are sent via digital carrier transmission lines from the central office reach a remote terminal unit before reaching the customer premises. The remote terminal then converts the digital signal to an analog signal, which may then be an intelligible signal for a telephone.
In the remote terminal, the digital data signals sent over the T1 lines are converted to analog telephone signals and then supplied to a customer loop telephone line. The analog telephone signals may then be received by the customer premises, which may include telephones and, in some cases, PBX units. Conversely, the remote terminals receive analog telephone signals from the telephones and PBX units and convert them to digital data signals that then can be transmitted over the T1 transmission system.
In operation of the T1 span equipment, there may be a ground fault such that, in effect, a lead is attached between one of the simplex transmission cables and ground. Such a fault may occur due to damage to a transmission cable. For example, construction digging may break or damage a transmission cable, or insulation coating the cable may deteriorate due to weather damage, animal damage, or other causes. Such a ground fault may result in signal loss or noise or errors being introduced into data transmitted along the cables between the central office and the network interface unit.
Occasionally, the central office may perform a test on various T1 span equipment, such as the repeaters, to determine whether direct current (d.c.) is being supplied, and thus whether or not there is a break in the simplex lines providing power to the repeaters. For example, the central office may sense current along the lines from the customer premises and the repeaters to the central office. If power is sensed along the receive line from the repeater, the central office determines that there is no break in the lines. In the case of a high resistance short, however, current is still present along the receive line, and the presence of this current may cause the central office to determine that there is no break in the lines. However, the current level along the receive lines is lower than the current supplied by the central office along the transmit line from the central office to the network interface unit. The system may fail to recognize this differential current and thus fail to report that a ground fault has occurred.